Can't use commands in v3.0.0preview-5

[Ayley]

I updated my project from 2.0.3 to 3.0.0preview- 5.
There was a lot of changes, I change everything so that I can start my app. But now I can't change my theme (using ApplicationThemeManager) and my RelayCommands don't executed when I click on a button. I didn't change the way I use commands between the versions.

What am I missing ?